TOUCH SWITCHES
TOUCH TRIGGERED BISTABLE     This circuit was devised as part of a touch controlled lighting system. It uses a 555 timer operated in the bistable mode.
Due to the high input impedance presented by the threshold and trigger terminals the 555 can be set and reset by the touch of a finger. Touching T2 causes the output to go high; D2 conducts and D1 extinguishes. Touching Ti causes the output to go low; D1 conducts and D2 is cut off.
    The output from pin 3 can also be used to operate other circuits e.g. a triac controlled lamp. In this case the LEDs are useful for finding the touch terminals in the dark.
    The capacitor C1 is not absolutely necessary but helps to prevent the circuit triggering from spurious pulses.
TOUCH-SENSITIVE SWITCH
 
    The circuit illustrated can be set to energise the relay when the plate is lightly touched. Under certain circumstances the proximity only of the body is sufficient to operate the switch.
    A high impedance input is provided by Q1, a general purpose field effect transistor such as 2N3819. A general purpose 741 op-amp is used as a sensitive voltage level switch and this in turn operates the current buffer 02, a medium current pnp bipolar transistor, thereby energising the relay which can be used to control equipment, alarms etc.
    In the quiescent state, the voltage at pin 3 of the op-amp is set higher than the voltage at pin 2 by adjustment of VR1. This ensures that the voltage at pin 6 is high and Q2 and the relay are off.
    Upon lightly touching the touch-plate, a decreasing reverse bias VGS increases the drain current flowing through Q1 and the resultant voltage drop across Hi lowers the voltage at pin 3 below that at pin 2.
    The voltage at pin 6 falls and switches on the relay via Q2. Resistor R4 may need to be selected to ensure that the relay is held off since a small positive voltage at the output remains even though the voltage at pin 3 is lower than that at pin 2 in the quiescent state.
    This problem can be overcome by using dual power supply for the op-amp in the more usual mode of operation of this device. Component values are not critical and there is considerable scope for experimentation.
    The sensitivity of the circuit to the proximity of the body depends upon the nature and strength of the surrounding electromagnetic fields produced by mains wiring and equipment in the vicinity, for it is the pick-up of this energy which the body couples to the circuit.